In the Linux kernel, the following vulnerability has been resolved: hwmon: (hp-wmi-sensors) Fix use-after-free in fungible_show() nsensor->current_state is dynamically replaced as the sensor's state changes

Overview

In the Linux kernel, the following vulnerability has been resolved:

hwmon: (hp-wmi-sensors) Fix use-after-free in fungible_show()

nsensor->current_state is dynamically replaced as the sensor's state changes. update_numeric_sensor_from_wobj() does this by freeing the old string and installing a new one:

if (strcmp(trimmed, nsensor->current_state)) {
	new_string = hp_wmi_strdup(dev, trimmed);
	if (new_string) {
		devm_kfree(dev, nsensor->current_state);
		nsensor->current_state = new_string;
	}
}

This function is only ever called from hp_wmi_update_info() while state->lock is held, so the free-and-replace itself is properly serialized against concurrent updates.

fungible_show(), however, reads the same pointer after the lock has already been dropped:

err = hp_wmi_update_info(state, info);
if (err)
	return err;

switch (prop) {
...
case HP_WMI_PROPERTY_CURRENT_STATE:
	seq_printf(seqf, "%s\n", nsensor->current_state);
	break;

hp_wmi_update_info() takes state->lock internally and releases it before returning, so by the time fungible_show() dereferences nsensor->current_state in seq_printf(), no lock is held. Two processes reading a sensor's current_state debugfs entry at overlapping times (or one reading it while another read of the same sensor triggers a refresh) can race: one thread's seq_printf() can be part-way through printing the string at the moment another thread's call into update_numeric_sensor_from_wobj() frees it with devm_kfree() and installs a new pointer, causing a use-after-free read.

Take state->lock around the read in fungible_show() as well, so it can never run concurrently with the free-and-replace in update_numeric_sensor_from_wobj().

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.